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Abstract

Spectral broadening of the fundamental field in intracavity Raman lasers is investigated. The mechanism for the spectral broadening is discussed and the effect is compared in two lasers using Raman crystals with different Raman linewidths. The impact of the spectral broadening on the effective Raman gain is analyzed, and the use of etalons to limit the fundamental spectral width is explored. It was found that an improvement in output power could be obtained by using etalons to limit the fundamental spectrum to a single narrow peak.

Figures (8)

Laser gain αF(ν0−δν) loss (including passive loss γF and loss due to the Stokes field Γ(ν0−δν)), and net gain αNET(ν0−δν) for wavelengths around the fundamental frequency
ν0, for an idealized single-longitudinal mode laser pumped at two
times the Raman pump power threshold. The laser gain and Raman spectrum are assumed to be
Lorentzian, with widths ΔνF and ΔνR respectively. In (a) the Raman linewidth is wider than the
laser linewidth with ΔνR/ΔνF=4/3 and in (b) the Raman linewidth is narrower than the laser
linewidth with ΔνR/ΔνF=1/3.

Schematic of BaWO4 laser cavity. DC was a plane dichroic mirror, highly
transmitting at the fundamental wavelength of 1064 nm and highly reflecting at the Stokes
wavelength of 1180 nm. M1 was a highly reflecting coating at the fundamental wavelength on
the back of the Nd:YVO4 crystal. M2 and M3 were highly reflecting at both
wavelengths, and had radii of curvature of 100 mm and −100 mm respectively. OC was a
concave output coupler with a radius of curvature of 250 mm and a transmission of 0.4% at
1180 nm.

Spectra of the BaWO4 Raman laser with (a) no etalons, (b) the 400 μm
thick YAG etalon and (c) both the 400 μm and the 50 μm thick etalons in the
fundamental cavity The upper plots (blue) show the fundamental spectra when SRS is
occurring. The lower plots (red) show the corresponding Stokes spectra. The absorbed pump
power was 31.5 W. Note the instrument linewidths are 0.09 nm for the fundamental and 0.06 nm
for the Stokes.